Principles of Ship’s Stability

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2 SHIP’S STABILITYSHIP’S STABILITY IS THE TENDENCY OF SHIP TO ROTARE ONE WAY OR THE OTHER WHEN FORCIBLY INCLINED

3 WHAY IS STABILITY IS SO IMPORTENT ?IF THE SHIP LOST STABILITY WHAT WILL BE HAPPENED:1. LOST OF MOBILE2. LOST THE HUMANS LIFES 3. LOST THE SHIP4. LOST THE CARGO5. OIL POLLUTION

4 FUNDAMENTALS OF STABILITYSTABILITY is the tendency of vessel to rotate one way or theother when forcibly inclined.IMPORTENT !!Ship’s stability can’t catch directlyStability can define only by calculating

11 STABILITYINITIAL STABILITY - The stability of a ship in the range from 0° to 7°/10° of inclination.OVERALL STABILITY - A general measure of a ship's ability to resist capsizing in a given condition of loading.DYNAMIC STABILITY - The work done in heeling a ship to a given angle of heel.

17 METACENTRIC HEIGHTMetacentric height GM is calculated by subtracting KGFrom KM (GM=KM-KG), GM is a measure of the ship.sstability. KM=h.With initial stability(0 – 10 deg.) the metacenter does notmove, and Sine function is almost linear(a straight line).Therefore, the size of the ship,s Righting Arm, GZ, isdirectly prportional to the size of the ship’s MetacentricHeight, GM.IMPORTENT !Thus , GM is a good measure of the ship’sinitial stability.

32 STABILITY CONDITIONSThe positions of Gravity and the Metacenter will indicate the initial stabilityof a ship.Following damage, the ship will assume one of the following three stabilityconditions:1. POSITIVE STABILITY. The metacenter is located abovethe ship’s center of gravity.As the ship is inclined, Righting Arm are created which tendto return the ship to it’s original, vertical position.2. NEUTRAL STABILITY. The metacenter and the ship’scenter of gravity are in the same location. As the ship is inclined, there are no returing moment.3. NEGATIVE STABILITY. The ship,s center of gravity isabove the metacenter.As the ship is inclined, negative Righting Arms (called upsettingarms) are created which tend to capsize the ship.

41 DYNAMIC STABILITYThe dynamic stability is the area under the curve in metre-radiansMultiplated by the ship,s displacement in tonnes. It is areas underthe GZCurve which are required for checking stability criteria whichdependingUpon the ship,s data may be expressed in metre-degrees ormetre-radians.The area unde GZ curve also the potential energy available toreturn theShip to the upringht.Principle of conservation of energy, the potential energyin converted intoRotation energy as the ship moves towards the upright.

44 Learning ObjectivesComprehend the concepts of hydrostatics, buoyancy, and Archimedes' principleComprehend static equilibrium of a floating vessel and the relationship of the centers of gravity and buoyancy to righting arms and stabilityComprehend and identify positive, negative and neutral conditions of stabilityComprehend the effects of movements of the centers of gravity and buoyancy on vessel stabilityKnow how ship's stability curves are derived and comprehend their use in determining stability condition

52 Buoyancy Archimedes' principle Calculations of displacement (W)The effect of salt water and fresh water on displacement (relate to draft)[1/35 vs 1/36]

53 Archimede’s principleBOYADA body immersed (or floating) in water willbuoyedARCHIMEDE’S FORCEBy a force equal to the weight of the waterdisplaced.

54 THE LAWS OF BUOYANCYFloatating objects posses the property of buoyancy.A floatating body displaces a volume of water equal ina body immersed (or floating) in water will be duoyedup by a force equal to the weight of the water displacedD=VgDLWGCVg

60 SHIP’S HULL MARKINGSNavigational Draft MarksShip’s operational drafts.These draft marks include the depth of any projections below the keel of the ship.Limiting Draft MarksLimiting drafts are assigned to maintain reserve buoyancy (freeboard) prior to damage, and to prevent excessive hull stresses as a result of overloading.

61 DISPLACEMENT GRAVITY MOMENTThe weight of the volume of water that is displaced by theunderwater portion of the hull is equal to theweight of the shipsGRAVITYThe force of gravity acts vertically downward through the ship’s centerOf gravity. The magnitude of the force depends on the ship’s total weight.MOMENTThe endency of a force to produce a rotation about a pivot point.This works like a torque wrench acting on a bolt.

68 DEFINITIONSCouple. Since the forces of buoyancy and gravity are equal and actalong parallel lines, but in opposite directions, a rotation is developed.Righting arm. The distance between the forces of buoyancy andgravity is know as the ship’s righting arm.Righting moment. The righting moment is equal to the ship’sRighting arm multiplied by the ship’s displacement.Metacentric height. The distance between center of gravity G andMetacener M .

69 - G does not change position as heeling angleThe development of the static stability curve from the cross curves of stabilityFoctors involed:- G does not change position as heeling anglechanges- C is always at the geometric center of the volumeof the underwater hull- the shape of the underwater hull changes asheeling angle changes

72 STATIC STABILITY CURVEWhen a ship is inclined through all angles of heel,and therighting arm for each angle is measured, the statical stability curve is produced. This curve is a “snapshot”of the ship’s stability at that particular loading condition.Much information can be obtained from this curve, including:Range of Stability: This ship will generate Righting Arms when inclined from 0 deg. Till to approximately 74 dg.Maximum Righting Arm: The angle of inclination where the maximum Righting Arm occursDanger Angle:One half the angle of the maximum Righting Arms.

73 DRAFT DIAGRAM AND FUNCTIONS OF FORMThe Draft Diagram is a nomogram located inSection II(a) of the Damage Control Book.It is used for determining the ship’s displacement, as well as otherproperties of the ship, including:- Moment to Trim One Inch (MT1);- Tons per Inch Immersion (TPI);- Height of Metacenter (KM);- Longitudinal Center of Flotation (LCF)- Longitudinal Center of Buoyancy(LCB)-Displacement (D)-VOLUME V m-Moment, diferenting per 1 cm-Weight, drafting per 1 cm

81 B – the ship’s beam to outside of hull. ROLLING PERIODThe rolling period of the ship’s dependenced from ship’s stability. The formulaBetween ship,s stability and rolling :T = c*B/sqr GMIn this formula:T – rollinperiod in sec.c - constantaB – the ship’s beam to outside of hull.Note: the constanta c dependenced from ship’s displacements.There are the followings meanings:c=0.88 – when ship is empty or ballast;c= when the ship has on board amout 20 %c=0.75 – when liquids on board 10%c=0.73 – when all liquids on board amout 5%HOWEVER, for all lagers ships Lloyd’s Register of shipping and the 1991 HMSOCode of Practice for Ro-Ro ships use c= 0.7